A Variety of Severe Weather in Southern New Jersey (credit: Colorado State CIRA, COD Meteorology)7/18/2017 
DISCUSSION: Conditions in the atmosphere this past Friday were aligned to produce dangerous weather for parts of southern New Jersey, seen above from space courtesy of GOES-16 via CIRA at Colorado State. Precipitable water values were as high as 2.3 inches in the area, allowing for excessive rainfall totals in any thunderstorms that formed. Bulk shear was as high as 45 knots, indicating that there was a rather high potential for thunderstorms to evolve into supercells. Skies became mainly clear during the afternoon to the south of Philadelphia, allowing for the lower atmosphere to become more unstable in that area. An approaching front from the northwest was the spark to put these ingredients to work and initiate the severe weather.
By 1:30pm local time, storms had exploded in southeastern Pennsylvania to over 50,000 feet tall. As this cluster moved slowly to the east-southeast, out ahead of it an isolated storm began to grow in the heart of southern New Jersey. This storm moved rather slowly and split in two after roughly half an hour, a phenomenon which is pretty common due to thunderstorm dynamics. Given the aforementioned bulk shear conditions, the right half of the split storm (formally known as a "right-mover") became a mature supercell with hail and rotation. You can see on the loop below, courtesy of the College of DuPage, that the supercell had a distinct "kink" feature to it as it moved southeastward towards Atlantic City. You can also see the left half of the original thunderstorm slowly fizzling just to the north of the supercell.
While no tornado warnings were issued, there was a rotating updraft evident from radar velocities as this storm also grew over 50,000 feet tall. I had the distinct pleasure of viewing the anvil of this supercell with some associated mammatus clouds. The storm weakened significantly by the time it reached me due to interaction with cooler marine air, but the constant thunder I could hear at the time of taking this video was pretty ominous.
At 3:30pm, as the supercell eventually moved offshore, the complex of storms from Pennsylvania had pushed into the Delaware Bay and was affecting towns along the extreme southern edge of New Jersey, such as Bridgeton and Millville. The rainfall rates were extraordinarily heavy, as Millville recorded 2.16 inches of rain in just 24 minutes. Using NOAA's Point Precipitation Frequency Estimates, this translates into an event that would occur on average just once every 200 years!
By 5:00pm, most of the action was off the coast or moving down through Delaware. However, a sudden flare-up of rotation and intense echoes occurred to the east of the southern New Jersey peninsula, as seen below on College of DuPage radar. The pink shading corresponded to echoes pushing 70 dBz, likely indicating hail. A look at radar base velocities showed clear and tight rotation within that area, meaning a waterspout was probably occurring at that time. Coastal residents were very lucky that this storm decided to intensify offshore and not half an hour earlier, or else seaside homes may have sustained notable damage.
From the isolated right-moving supercell to the amazing rainfall rates in Millville to the possible offshore waterspout and hail, it was an interesting and unusual day of weather for southern New Jersey. Some of the conditions necessary for these types of storms to occur simply aren't common in this area. However, this event served as a great reminder that severe weather can occur anywhere at anytime, and that you should take proper steps to be prepared for impacts such as a power outage or a flooded basement.
To learn more about other severe weather topics from around the world, be sure to click here! © 2017 Meteorologist Jake Spivey
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